The present invention relates to a light-amount detecting device which detects an amount of light of a light beam emitted from a light source, to a light source device provided with the light-amount detecting device, to an optical scanning unit having the light source device, and to an image forming apparatus provided with the optical scanning unit.
Conventionally, as an image forming apparatus which forms an image by utilizing a Carlson process, there has been known, for example, an image forming apparatus which forms an image by scanning a surface of a rotating photoreceptor drum with a light beam to form an electrostatic latent image on the surface of the photoreceptor drum, visualizing with a toner the electrostatic latent image as a toner image, and fixing the toner image onto a paper as a recording medium. In recent years, such a kind of image forming apparatus has been often used for a simplified printing as an on-demand printing system, and thus demands for attaining a high-density of an image and speed-up of an output of the image are being increased.
As a method of realizing the high-density of the image and the speed-up of the image output simultaneously, Japanese Patent Registration No. 3227226 proposes an image forming apparatus in which a light source emitting a plurality of light beams is used to scan photoreceptor drums with the light beams at once. The proposed image forming apparatus is possible to simultaneously scan the photoreceptor drums with the plurality of light beams, by collectively deflecting the light beams emitted from a surface emitting type laser (VCSEL: Vertical Cavity Surface Emitting LASER) array, having a plurality of light-emitting points, with a polygon mirror.
In addition, in an image forming apparatus, a light beam emitted from a light source influences quality of an image. Accordingly, various measures have been proposed in order to obtain the stable light beam.
For example, Japanese Patent Publications No. 2006-332142, No. 2006-91157, No. 2005-156933, and No. 2006-259098 each discloses an optical scanning unit which automatically changes an amount of driving current of light-emitting elements according to a change in light intensity of the light-emitting elements, by feeding back the light intensity of the light-emitting elements.
In addition, there are possibilities that an amount of light of a scanning light beam, which scans a photoreceptor drum, is changed in accordance with a temperature change, a secular change and so on, and that density unevenness is thereby generated on an image to be formed. In order to suppress such possibilities, normally, an optical scanning unit of an image forming apparatus detects an amount of light of light emitted from a light source or detects intensity of the light with a sensor such as a photodiode, to implement an APC (Auto Power Control) which controls the light-amount of the light source or the intensity of the light, on the basis of a result of the detection by the sensor.
In an image forming apparatus utilizing an optical scanning unit that detects a light-amount of a light beam emitted from a light source to control the light source, however, there is likely that returning light or reflected light of the light beam, utilized for the detection of the light-amount of the light source enters or incident on the light source. When the returning light or the reflected light is incident on the light source, laser oscillation may become unstable, and thereby, generation of noise and decrease in a laser output may be incurred.
In addition, in the image forming apparatus utilizing the optical scanning unit, light-emitting portions (or light-emitting elements) are lit constantly while the image forming apparatus is in operation. Hence, there is a possibility that deterioration in the light-emitting portions is occurred dues to heat generation of the light-emitting portions.
Moreover, it is desirable that the optical scanning unit be provided with a structure in which a positional relationship between the light source and an optical system is easily adjustable.